During a process for the fabrication of an integrated semiconductor circuit, a plurality of layers of different materials are applied to a semiconductor substrate, for example a silicon wafer. The applied layers are generally patterned. The patterned layers either remain in place and serve as electrically conducting layers, insulation layers or passivation layers in the finished semiconductor circuit, or are temporary in form and are used as a mask or starting layer for doping processes as part of the fabrication process.
On account of a layer material which forms one layer and the material of a layer beneath it or the semiconductor material of the semiconductor substrate having different coefficients of thermal expansion, stress relief layers, which reduce and/or compensate for mechanical stresses resulting from the different coefficients of thermal expansion and/or convert such stresses into a compressive pressure on the layer below, thereby preventing the layer on top from flaking off, are to be provided between the layer below and the layer on top for some combinations of layers.
It is therefore known for example to provide a silicon dioxide layer with a layer thickness of a few nanometers as stress relief layer between a monocrystalline silicon substrate and a relatively thick silicon nitride layer, with a layer thickness of more than 50 nanometers, above the monocrystalline silicon substrate. Silicon nitride layers are preferred for use as mask material or part of a mask on account of their good barrier properties with respect to diffusion phenomena of all types and on account of the fact that this material has a relatively high resistance to a range of etching processes that act on silicon.
One typical example relates to a mask for forming trenches in order to produce storage capacitors for memory cells of dynamic random access memories (DRAMs) in a semiconductor substrate. In this context, a silicon nitride layer (pad nitride), beneath which there is a stress relief layer (pad oxide) of silicon dioxide, protects covered portions of a semiconductor substrate both from the etching process for forming the trenches and from a subsequent extensive processing affecting the trenches in order to form the storage capacitors.
The etching processes which are of relevance to the invention as part of the formation of a trench in a semiconductor substrate are illustrated with reference to FIG. 1.
FIG. 1A shows a mask 3 and a stress relief layer 2 formed from silicon dioxide beneath the mask 3, through which a trench 4 has been introduced into a semiconductor substrate 1 by means of a trench etch. An original hard-mask portion 3a of the mask 3 made from borosilicate glass has been consumed apart from a hard mask residue 3a′. During the trench etch, a silicon oxide coating 3c was deposited on the vertical wall of the trench 4, assisting targeted etching of the semiconductor material into the depth of the semiconductor substrate 1.
After the trench etch has ended, the silicon oxide coating 3c which has been deposited at the trench wall is removed selectively with respect to the silicon of the semiconductor substrate 1 by means of a cleaning etch step. In the region of the semiconductor substrate 1, the cleaning etch step stops at the silicon of the semiconductor substrate 1. Since the cleaning etch step is controlled for a sufficiently long time to ensure reliable removal of the silicon oxide coating 3c, the cleaning etch step, after the silicon oxide coating 3c has been removed, also acts, in the region of the edge which is then uncovered, on the stress relief layer 2, which is likewise formed from silicon dioxide, and causes the stress relief layer 2 to recede. The hard-mask residues 3a′ are then to be removed by a receding etch step. A material containing silicon oxide, such as borosilicate glass, is used for the hard-mask portion 3a, so that the receding etch step also acts on the silicon dioxide of the stress relief layer 2 and the stress relief layer 2 is caused to recede further.
Following the cleaning etch step and the receding etch step, the result, in simplified form, is the structure illustrated in FIG. 1B. The silicon oxide coating 3c and the hard-mask residues 3a′ have been completely removed. Undercuts 5 have been formed in the stress relief layer 2 proceeding from the trench structure 4 beneath the silicon nitride layer 3b above it. During the further processing, the trench structure 4 is filled, for example with a conductive material, to form an inner electrode of a storage capacitor. The conductive material fills both the trench structure 4 and the undercuts 5, with the result that disruptive conductive structures are formed on the process surface 10 outside the trench structures 4. To restrict the extent of the undercuts 5, the process time for the cleaning etch step has hitherto been kept short and a hard-mask material which is relatively easy to remove has been used, so that the process time for the receding etch step can also be kept short.
U.S. Pat. Nos. 6,461,937 and 5,447,884 have disclosed a silicon nitride layer as stress relief layer between a layer of thermally grown silicon oxide, on the one hand, and silicate glass applied by means of a vapor deposition process (CVD, chemical vapor deposition), on the other hand, during the pr oduction of a trench isolation region. In this case, an isolation trench is introduced into a semiconductor substrate through a silicon nitride mask with a stress relief layer of silicon oxide beneath it. The isolation trench is lined by thermally grown silicon dioxide and is then filled with undoped silicate glass deposited by CVD. Since the semiconductor substrate has a coefficient of thermal expansion which differs significantly from that of the undoped silicate glass, considerable mechanical stresses are produced in the region of the isolation trench during further processing.
Therefore, a thin silicon nitride layer is provided as stress relief layer which is applied before the undoped silicate glass is deposited on the thermally grown oxide.
Another drawback of masks with a silicon nitride layer and a stress relief layer of silicon dioxide beneath them is that the mask and the stress relief layer have to be removed in two steps involving a change in the etching process. The stress relief layer is used as the etching stop layer or etching stop signal layer for removal of the silicon nitride layer above it.